1. Field of the Invention
The present invention relates to a polymer useful for positive photoresist and a positive photoresist composition containing the same.
2. Description of the Prior Art
The high integration of semiconductor devices has always been followed by significant advance in lithography. For example, ultra-fine patterns as small as sub-microns or quarter-microns in size must be required for the fabrication of ultra-LSI. Accordingly, as the light sources used to form the fine patterns become shorter in wavelength from g-line to i-line and deep uv light, further to excimer laser, such as a KrF laser, and finally to electron beams, the development of the compositions corresponding to the light sources are essential for the lithography.
Since the photoresists prepared from novolak-quinone diazides, materials used for g-line or i-line, show large absorption at the wavelength range of deep uv light and excimer laser, a fine pattern cannot be obtained from the photoresists. Thus, there was a strong demand for a material that little absorbs the light belonging to such wavelength ranges. In response to the demand, active research has been directed to the development of chemical amplified photoresists based on polyhydroxystyrene derivatives which are smaller in absorption at 248 nm than novolak-quinonediazides.
Chemical amplified photoresists are characterized in that they show high sensitivity and resolution and the physical properties of their matrix resins are changed by a catalytic amount of the acid which is generated upon radiation. For example, a photoresist prepared from a matrix resin in which the hydroxy groups of polyhydroxystyrene are partly replaced with t-butylcarboxy groups and an onium salt, a compound which generates acid upon radiation (hereinafter referred to as "photoacid generator"), is disclosed in U.S. Pat. No. 4,491,628. Another example is a resist employing a copolymer of 4-hydroxystyrene and t-butylacrylate as a matrix resin and a sulfonate compound as a photoacid generator, as disclosed in J. Photopol. Scien. and Tech., 9, 557-572, 1996. However, these conventional resists are thermally instable and their polymers are difficult to synthesize. Further, in the case of the polyhydroxystyrene, it may be oxidized into quinone compounds, which absorb much light at 248 nm.